(1) Field of the Invention
The present invention relates to a magnetic toner wherein a toner particle contains the magnetic particle and a binder resin and further the two component developer contains the toner particle and a carrier. Further, the present invention relates to the electro-photographic image forming method using the two component developer.
(2) Description of the Prior Art
In the electro-photographic image forming apparatuses such as laser printers, copiers, facsimiles and their hybrid apparatuses, first, the electrostatic latent image retaining member, i.e., a photoreceptor is electrically charged uniformly, second, the photoreceptor is exposed, thereby forming the electrostatic latent image, third, the latent image is developed by the toner, forth, the toner image is transferred onto the paper, or the intermediate transfer member, and finally the transferred toner image is fixed onto the paper.
Although there are dry and wet developing methods, the dry development is more widely employed than the wet development. Further, the dry developer is classified into two types. One of them is the magnetic toner wherein the magnetic particles are added internally or externally to the binder resin for the single component or two component developer. Other is the non-magnetic toner wherein the magnetic particles are not added.
Particularly, in the two component developer, the minute toner particles are attached to the relatively large-sized carrier particle such as iron or ferrite particle. The two component developer is carried to the photoconductor. When it comes near the latent image on the photoconductor, the toner particles are attached onto the photoconductor by the attractive force due to the latent image electric field which is stronger than the toner—carrier attractive force.
Further, the two component magnetic developer has an advantage that the toner scattering, the toner dropping and the fogging on the photoreceptor can be avoided, because the toner is attracted to the carrier by the magnetic force. Further, the two component magnetic developer has another advantage that the excessive charging of the magnetic toner due to the repetitive mixing of the magnetic toners and the carrier can be avoided by the charge leak from the magnetic particles.
Recently, the image forming process speed becomes farther higher and the image forming apparatus is made farther smaller-sized.
Particularly, the high speed apparatus for the business use should prevent the degradation of image resolution and quality. Accordingly, it is required to more rapidly rise up the electric charge quantity of the magnetic toner and moreover to stabilize the electric charge quantity.
Further, the smaller-sized middle and low speed apparatus for the small office and family use should farther shorten the warming-up time after switching on the apparatus, because the electric power supply is very often switched on and off. Accordingly, it is required to well and rapidly charge the magnetic toner when the power supply is switched on.
Further, regardless of the image forming speed, it is required to farther improve the image quality & resolution and the endurance & environmental stability of the magnetic toner.
However, the conventional magnetic toners generally used are not satisfactory in the current tendency seeking higher process speed and smaller-sized apparatus.
The magnetic particle is, in general, a sphere (see FIG. 1A) or a polyhedron such as a cube, a hexahedron (see FIG. 1B) or an octahedron (see FIG. 1C). Such a magnetic toner is tends to leak the electric charge, because the electric charge is easily leaked from the vertexes and edges of the polyhedron. Further, the magnetic polyhedrons do not well disperse.
Accordingly, the polyhedron magnetic toner has a disadvantage that the charge quantity does not rapidly rise up, resulting in a low charge quantity, the toner scattering & dropping and the image quality degradation such as the fogging in the image background. Further, it has another disadvantage that the image quality is farther degraded in the high temperature and high humidity environment where the electric charging becomes difficult.
On the contrary, the spherical magnetic particle has an advantage that the charge leak is not easily induced, because it does not have any vertex nor edgeline. Further, it has another advantage that it disperses in the binder resin more than the polyhedron particle, thereby preventing the fluctuation of the dispersion, the easiness of the electric charging and the charge quantity.
However, the spherical magnetic particle is easily charged so much that the charge-up is easily caused and the image quality degradation such as the low image density is easily induced.
Therefore, the various shaped magnetic particles are studied, in order to take in the shape merits of the sphere and polyhedron.
For example, there are disclosed in JP11-153882 A (1999), JP2000-162817A and JP2000-242029A that the vertexes and edges of polyhedron are trimmed. However, pointed lines between the trimmed surfaces and the original polyhedron surfaces are left yet, thereby easily discharging from the pointed lines and therefore thereby possibly causing the low image density and the background fogging.
Further JP9-59024A, 1997 discloses that the edgelines of the polygon are rounded. However, it has a disadvantage that the magnetic toner is charged up similarly to the spherical magnetic toner, thereby possibly causing the image quality degradation.
Further, JP56-106249A (1981) and JP59-162563 (184) assert that the two component magnetic developer suppreses the toner scattering and the background fog. However, they does not disclose the endurance characteristics in the multiple times of the repetitive image forming processes. Accordingly, the electric charge quantity may not be stable after making a quite lot of copies.